1. Field of the Invention
The present invention relates to the field of digital signal transmission and to the detection of bipolar line coding violations in digital signal transmission.
2. Description of the Prior Art
As is well known in the prior art, a single PCM channel known as a DS0 channel operates at 64 Kb/sec in each direction of transmission to transmit and receive 8,000 8-bit samples per second of a desired telecommunication transmission, whether voice or data. In accordance with Bell standards, individual bidirectional channels are multiplexed into higher speed channels for long distance transmission. For example, twenty-four DS0 channels may be multiplexed into a DS1 channel or hierarchial level operating at 1.544 Mb/sec. In accordance with this format, 24 8-bit samples, one from each DS0 channel, are arranged serially in a single transmission frame together with a single framing bit to form a 193-bit frame. The bit rate of 1.544 Mb/sec. is arrived at by transmission of successive 193 bit frames at a transmission rate of 8,000 frames per second.
DS1 signal transmission of the type described hereinbefore is typically line coded in a bipolar line coded format which bipolar coding is a form of ternary coding in which the signalling rate is equal to the input binary rate. Binary "ones" are represented by either positive or negative pulses and binary "zeros" are represented by the absence of a pulse. In bipolar coding format the polarity of the signal level is changed at each occurrence of a logical one. Errors in transmission whether pulse deletion or pulse addition result in a coding violation called a bipolar violation (BPV). For example, two positive level ones or two negative level ones in a row would constitute a bipolar violation and would be indicative of an error. Bipolar line coding and error detection is well known in the prior art and reference can be made to standard reference material for a more detailed description thereof, such as Transmission Systems for Communications, Fifth Edition, Bell Telephone Laboratories Inc. 1982 at pages 742 through 746 thereof.
In order to properly correct the aforementioned bipolar violations which occur in bipolar line coding, it is necessary to monitor the line status during transmission to measure and count the occurrences of such bipolar violations and to raise alarms when the error rate exceeds certain threshold criteria, known in the telecommunications transmission field as service affecting and non-service affecting thresholds, so that appropriate remedial action can be taken if necessary. Typically a service affecting threshold would be the cause of a major alarm and might represent one error in each 1,000 bits of transmission; while a non-service affecting threshold level of bipolar violation would cause a minor alarm and might be indicative of an error transmission rate of one error in each million bits of transmission.
In accordance with the present invention, an improved technique for calculating bipolar error violations is disclosed.